The present invention relates to a process for preparing polyisocyanates from natural raw material sources, in which a composition comprising low molecular weight aromatics which comprise at least one hydroxy group or at least one alkoxy group per molecule (oxyaromatics) is produced from a biomass material, these oxyaromatics are converted into the corresponding aromatic amines and, optionally after condensation with formaldehyde, reacted further with phosgene to give compounds comprising isocyanate groups.